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- Water Activities (8)
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Water activity
From Wikipedia, the free encyclopedia
Water activity or aw is the relative availability of water in a substance. It is defined as the vapor pressure of water divided by that of pure water at the same temperature; therefore, pure distilled water has a water activity of exactly one.
As the temperature increases, aw typically decreases, except in some salt and sugar solutions.
Higher aw substances tend to support more microorganisms. Bacteria usually require at least 0.91, and fungi at least 0.7. See fermentation.
Water tends to migrate from high aw substances to low aw substances. For example, when honey (aw ā 0.6) is exposed to humid air (aw ā 0.7) the honey will absorb water from the air.
Contents |
Definition of aw:
where p is the vapor pressure of water in the substance, and pā is the vapor pressure of pure water at the same temperature.
Alternate definition:
where lw is the activity coefficient of water and xw is the mole fraction of water in the aqueous fraction.
Estimated mold-free shelf life in days at 21Ā° C:
| Substance | aw |
|---|---|
| Distilled Water | 1 |
| Tap water | 0.99 |
| Raw meats | 0.97 - 0.99 |
| Milk | 0.97 |
| Juice | 0.97 |
| Cooked bacon | < 0.85 |
| Saturated NaCl solution | 0.75 |
| Point at which cereal loses crunch | 0.65 |
| Typical indoor air | 0.5 - 0.7 |
| Honey | 0.5 - 0.7 |
| Dried fruit | 0.5 - 0.6 |
- Fennema, O.R., Ed. (1985). Food Chemistry - Second Edition, Revised and Expanded. New York: Marcell Dekker, Inc. pp. 46-50.